The studies comprising this proposal are directed at determining the structure and expression of alpha, beta and gamma actin genes during myogenesis. The first requisite step for these studies is the development of probes specific for the different actins. Toward this end, I have cloned cDNAs from pre- and post-fusion cultures of the rat myoblast cell line L6. This diploid tissue culture cell line retains the ability to undergo fusion and provides a homogeneous population of muscle cells for study. Characterization of the recombinant cDNA clones is in progress. Alpha, beta and gamma specific recombinant probes will be used to study the expression of actin genes during development. Experiments will be carried out to determine in pre and post fusion cells 1) the content and stability of actin RNA sequences, 2) the pathways by which these sequences are processed into mature mRNAs, and 3) the structure and organization of alpha, beta and gamma actin genes. these studies will determine whether transcriptional or post-transcriptional mechanisms are responsible for regulated expression of muscle (alpha) and non-muscle (beta,gamma) actin sequences during development.